Consider making CondaPkg a weak dependecy

As I believe that it is not necessary when `JULIA_CONDAPKG_BACKEND=Null`. Relevant for Julia >= 1.9.

Start by locating the package dependency declarations and the handling of JULIA_CONDAPKG_BACKEND=Null, then check Julia 1.9's weak-dependency support. Done means CondaPkg is optional for the Null backend while the existing CondaPkg-backed behavior continues to work.

由索引模型根據 Issue 內容生成。